Winter ulcer is a known and only partly solved problem within the salmonid farming industry, especially relevant for the Atlantic salmon and rainbow trout farming. Winter ulcer causes annual losses of around NOK 100 million to the Norwegian aquaculture industry. In addition new findings show that more than 7 million salmons already vaccinated against winter ulcer are potentially lost to the industry only during the first 3 months after sea transfer due to ulcers that often is demonstrated to be winter ulcer. Industry officials have also identified a substantial lack of reporting of the illness, as the reporting of this disease is not mandatory. Out of the NOK 100 million registered losses, NOK 20 million are caused by down-classification of fish products due to scars and damaged muscle tissue which again causes a lowered meat quality.
Winter ulcer has also been a problem to the aquaculture industry in Scotland, Iceland, Faroe Islands, Ireland, Canada and Maine in USA (Bruno et al. 1998, Gudmundsdottir et al. 2006, Whitman et al. 2000).
Moritella viscosa (previously called Vibrio viscosus) has since long been shown to cause winter ulcer in farmed salmonid fish. From the first known outbreaks of disease in the 1980s focus has been on preventing and controlling the disease by controlling the bacterium M. viscosa. Since 1993, fish have been vaccinated against winter ulcer, and the main strategy today is to use a multi-component vaccine in vaccination of smolts before sea launch. M. viscosa is one of the up to six microbial components in the multi-component vaccines from all the vaccine producers. Other components besides M. viscosa are intended to protect against other diseases than winter ulcer. Vaccination of farmed Atlantic salmon has for nearly three decades been performed with one single intra-peritoneal injection of a multi-component vaccine protecting against up to 6 different infectious diseases before smoltification and sea launch. Since vaccination against winter ulcer started the vaccine effect has not been optimal, being successful with ≧60% relative protection.
Aunsmo et al. in 2008 document ulcers as accounting for 43% of the mortality in Atlantic salmon smolts already vaccinated against winter ulcer during the 3 first months after transfer to sea. It was shown that ulcers occurred in outbreaks with both M. viscosa and V. wodanis isolated from the kidneys of vaccinated but diseased salmon. In addition ulcers occurred as a cause of low baseline mortality during the first 3 months. Further winter ulcer occurs as a problem in all parts of the growth period.
The diseased post-smolts will also have a lower chance of becoming fully grown salmons having a higher mortality rate. The study published by Arnfinn Aunsmo in his 2009 Ph.D. thesis, entitled “Health related losses in sea farmed Atlantic salmon-quantification, risk factors and economic impact”, at the Norwegian School of Veterinary Science (ISBN 978-82-7725-168-4, h.) (Aunsmo et al. (2008) had followed 2.7 million vaccinated post-smolts after transfer to sea in 20 different cages at 10 different localities in Norway. Overall mortality of the smolts was about 2.5% with the main cause of mortality explained by wounds (mainly winter ulcer) accounting for 43% of deaths in spite of the use of the only relatively effective commercial multi-component vaccine with the winter ulcer component. Extrapolated, 2.5% of the about 280 millions of salmon smolts ‘launched’ in total annually in Norway die from wounds that primarily are caused by winter ulcer the first 3 months after transfer from freshwater to net pens in sea.
Winter ulcer occurs throughout the complete growth period except for the summer months and the total losses including loss of salmon near to slaughter is not known. However, in 2008 it was reported a direct loss from down-classification of slaughtered salmon of NOK 20 million suspected to be mainly caused by winter ulcer in only one single salmon farm (ref Fiskehelserapporten VI, 2008).
Winter ulcer is a disease not mandatory to report, but by an annual phone call to the local Fish Health Services made by an official at the National Veterinary Institute annually between 35 and 55 farms are “remembered” to have had outbreaks of winter ulcer by the various Fish Health Services. In the fish health report from the National Veterinary Institute covering 2010 it is noted that a “Smolt syndrome” has been recorded just after transfer to sea. The smolts grow poorly and develop ulcers and it is related to improper smoltification in large batches of salmon transferred to unusually cold water. It is reason to believe that winter ulcer bacteria together with Tenacibaculum spp. bacteria may be related to these ulcer problems.
In summary, there is a need in the art to overcome or at least mitigate the problems associated with disease in fish, such as Salmonidae, by finding alternative vaccine solutions to the vaccines available as of today. There is a further need in the art for improvements of the vaccines to winter ulcer. There is a further need in the art for an improved vaccine which will reduce the loss caused by ulcers in the salmonid farming and also improve the product quality due to the occurrence of reduced scars and connective tissue in the meat of salmon surviving winter ulcer especially in spring and summer when sea water temperatures rise and ulcers heal leaving scars.